Scheduled to open May 3, the case was twice postponed, from Jan. 4 to March 15, and then to May. It could be delayed again. There is continuing legal maneuvering by the defense to prohibit the government's use of wiretap evidence, considered important by the prosecution.

Two of three Teamsters presidents before Mr. Williams were tried, convicted, and sent to prison - Dave Beck in the 1950s for embezzlement, larceny, and income tax evasion, and Mr. Hoffa for jury tampering, fraud, and conspiracy in connection with administering pension funds.

Organized labor is considered by labor experts to be relatively free of large-scale corruption. Since the early 1970s, however, more than 400 union officials and employees have been convicted of labor-related crimes. Most were local cases; only a few involved high-level union officers.

In the current case involving Williams, named Teamsters president in June 1981 despite federal charges against him, the Justice Department alleges he conspired to bribe Sen. Howard W. Cannon (D) of Nevada by offering him a chance to buy Nevada land at an advantageous price in return for support in blocking trucking deregulation. Senator Cannon refused the deal.

About 2,500 reels of wiretap tape recordings were made by 20 FBI agents during a 141/2-month investigation. One federal prosecutor said recently, ''The tapes are essential. There can be no case if they are supressed.''

The Teamsters president and his associates have pleaded innocent.

One month ago, the Justice Department moved in a different direction against the Teamsters when it filed a civil action against the union's second-largest local in Union City, N.J., in an effort, the Justice Department said, ''to root out corruption that has been going on for 20 or 30 years.''

The suit, the first of its kind against a union, was brought under the US Racketeer-Influenced and Corrupt Organizations Act. It seeks to have trustees named to end ''systematic union corruption.'' The local is known as the ''Provenzano local,'' for the three brothers who have headed it - Anthony (Tony Pro) Provenzano, long a major power in the Teamsters and now serving 20 years in prison for racketeering; Nunzio Provenzano, sentenced to 10 years for extortion; and Salvatore Provenzano, aquitted on racketeering charges in 1981.

In addition to Anthony and Nunzio Provenzano, convicted labor leaders have included Joseph Tonelli, a top union official who was also a member of the AFL-CIO executive council; Anthony M. Scotto, an International Longshoremen's Association vice-president sentenced to five years for racketeering; and a former Teamsters vice-president, William Murphy, who was found guilty of taking gifts illegally.